Phase locked loops, particularly for mobile radio applications, are important components of large-scale integrated circuits. Such phase locked loops for providing stable-frequency signals are known, by way of example, from Tietze/Schenk: “Halbleiterschaltungstechnik” [Semiconductor Circuitry], pp. 1155–1156, 12th Edition, Springer 2002 and Mike Curtin et. al. “Phase-locked Loops for High-Frequency Receivers and Transmitter—Part 2”, Analog Dialogue 33–5, Analog Devices 1999.
When implementing large-scale integrated circuits, the problem of very large tolerances in the component parts of the individual switching elements frequently arises. These are in some cases process-related and are extremely difficult to calculate in advance during a development or design phase. It is therefore usual to take into account the parameter discrepancies in the components in integrated circuits during an actual development phase and to plan on an over dimensioned scale if appropriate.
In mobile radio applications, phase locked loops are used to provide an oscillator signal. These phase locked loops are used to actuate a voltage-controlled oscillator, with the control signal being supplied to the setting input of the voltage-controlled oscillator via a filter. In phase locked loops, this filter is often a determining variable for the dynamic response of the whole control loop. It is therefore of great importance to ensure a particular and easily calculatable filter characteristic. Discrepancies from an ideal filter characteristic on account of component fluctuations need to be restricted as far as possible in the circuitry.
Besides the possibility of compensating for the aforementioned component tolerances in the development phase of the phase locked loop by means of appropriate over dimensioning features, another option is to trim particularly the RC elements which determine the filter characteristic. In this context, these elements are often arranged outside of the circuit in order to allow trimming in a later production step too. However, this results in an additional space requirement which conflicts with the demand for miniaturization and large-scale integrated circuits.
Trimming is sometimes carried out using an internal reference element. The reference may likewise be in the form of a large-scale integrated circuit. However, this also requires additional space, since particularly the capacitors which contain the RC element take up a relatively large surface area in semiconductor processes today. Furthermore, additional current and voltage sources are required for operating the reference element.